The present invention provides systems and methods for obtaining helium-rich product fractions from feed streams containing carbon dioxide and at least one of nitrogen and methane.
There are many high-pressure gas fields that supply carbon dioxide-rich gas streams for the oil and gas industry. In general, the carbon dioxide (CO2) content of these streams is greater than 50% by volume, and usually from about 60 to about 98 vol %. In addition, the gas mixture typically contains methane (for example from about 0.1 to about 20 vol %), nitrogen (up to about 30 vol %), small amounts of argon, hydrogen, and heavier hydrocarbons, and helium fractions up to about 1% by volume. These CO2-rich gas streams have been used in the industry for enhanced oil recovery (EOR), and the associated hydrocarbons are optionally recovered when economically justified.
Helium is used in a variety of applications, including for example cryogenic processes, pressurizing and purging systems, maintaining controlled atmospheres, and welding. Since helium is becoming increasingly scarce, however, new ways to recover helium are being considered, including recovering the small amounts of helium contained in such CO2-rich streams. In order to do so, a product stream must be recovered that has a sufficient composition and pressure for further treatment in a helium purification and liquefaction process. The recovered helium-rich product fraction, then, should have a helium content of at least about 35 vol % in nitrogen, preferably greater than 50 vol %, with only trace amounts of CO2.
Because the triple point of CO2 is −56.6° C., purification processes that rely on phase separation carried out by means of a low temperature distillation process can perform only a coarse separation of the CO2 from a feed gas mixture. CO2 separation can also be carried out by other methods, such as amine scrubbing, methanol scrubbing, or adsorption processes such as pressure swing adsorption (PSA), but an improved process that maximizes recovery of helium and CO2 and minimizes power requirements is commercially and economically desirable.
U.S. Pat. No. 3,653,220 describes recovery of helium from a natural gas-containing feed in which CO2 in the feed (approximately 22 vol %) is removed using a CO2 PSA upstream of a low temperature separation unit. Recovering helium from a natural gas feed, however, is simpler than recovering helium from a feed having greater than 50 vol % CO2, because cryogenic separation may be employed earlier in the recovery process to produce pure helium.
U.S. Pat. No. 3,740,962 is similar to the previous process, except that CO2 is removed from a natural gas feed using an amine scrubbing unit, and crude helium is then separated from a predominantly methane stream via a cryogenic separation process using external refrigerants to condense the feed gas.
WO 2010/060533 describes bulk separation of CO2 from a helium-containing natural CO2 source using a low temperature separation process, with further downstream helium purification. DE 102008007925 describes recovery of helium from a feed containing helium, nitrogen, and methane by partial condensation of the feed and subsequent separation into helium-rich and helium-lean fractions.
The present invention provides an improved process for recovering helium from a CO2 feed while preferably recovering at least 98% of the helium in the feed to the helium product, preferably recovering at least 99% of the CO2 in the feed back to the pipeline, minimizing power requirements, and meeting the CO2 composition requirements of a downstream pipeline by optionally rejecting nitrogen within the helium recovery process.